When a teleconferencing system or remote lecture system is implemented using a communication network, it is necessary for the same information to be distributed from an unspecified terminal to a plurality of terminals (hereinafter referred to as “multipoint communication”). For example, in the case of a remote lecture, an unspecified student asks a question to a lecturer. Consequently, a remote lecture system must distribute not only a lecturer's speech information but also speech information of a student asking a question to a plurality of remote lecture participants.
IP (internet protocol) multicasting is known as a technology that implements multipoint communication. However, with IP multicasting it is necessary for all routers in a communication network to support IP multicasting. Therefore, it is difficult for IP multicasting to be applied to an existing or general-purpose communication network such as the Internet. That is to say, with IP multicasting it is difficult to implement multipoint communication cheaply and flexibly.
ALM (application layer multicast) is known as another technology that implements multipoint communication. ALM is one kind of overlay multicasting. ALM is a technology whereby the same information is distributed to a plurality of terminals by having each terminal duplicate a received packet and transfer it to another terminal in accordance with a predetermined distribution path. Since packet duplication and transfer is controlled in the application layer, ALM is easily applied to an existing or general-purpose communication network.